Eggs are special cells found in female animals. Nearly all animals produce eggs,
and some animals lay their eggs outside their body. Birds lay eggs that have a
hard outer covering or shell to protect the baby inside. Inside the shell are
the egg white (albumen) and the yellow egg yolk, which are used as food for
the developing baby chick.

The eggs that we eat (usually hen's eggs) are not fertilised, and so a baby
bird does not grow inside them. If male and female birds are allowed to mate,
some of the mother's eggs will be fertilised and a baby bird can develop inside
them. First, starting before the egg is even laid, a small part of the yolky
bit of the egg divides into smaller parts, or cells, that will make up the new
baby. By the time the egg is laid, a few hundred tiny cells make up the beginnings
of a baby chick (embryo), which is just a tiny flat white dot on the yolk. To help
her baby grow, the mother hen has to sit on the egg to keep it nice and warm.

Some of these cells turn into the backbone of the chick, others will make the
heart, yet others will form the brain and eyes. Slowly, over the next few days,
the baby chick grows, adding more and more cells. Blood vessels form across the
yolk, allowing the food in the egg to be used as energy to make the baby chick grow.
After 3 days the baby chick begins to sprout wings and legs, and after 12 days feathers
begin to grow. After 3 weeks, the chick has grown so much that it fills the whole shell
and has run out of food and air. It first makes a small hole in the shell with its beak
and then pecks at the shell until it can hatch into a baby chick.